A manufacturing method for a pulley made of sheet metal for toothed belts is disclosed in Japanese Patent Laid-Open, No. 177457/1982, which has the disadvantages that for toothed pulleys, the wall thicknesses of both shoulder sections in the pulley tooth section are extended by 60% of their material plate thickness so that a perfect tooth shape is difficult to achieve and the rigidity required for the tension in toothed belts is likely to be insufficient.
An object of the present invention is to solve the above-mentioned disadvantages related to the tooth shape and to the rigidity of conventional pulleys made of sheet metal for toothed belts by simultaneous cold forging from sheet metal of pulleys for toothed belts.
A pulley in this invention is characterized in that the teeth of a pulley for toothed belts are formed by a single cold forging of a metal sheet so as to have a plurality of reinforcing cross-pieces or reinforcing ribs arranged in the circumferential direction which are integral with the pulley and project from the inner surface of the metal sheet which forms the teeth.
Furthermore, the method of manufacturing a pulley according to the invention uses a cup-shaped body which has a generally circular base with a circumferential wall section or upstanding flange which is to form the tooth section of a sheet metal pulley for toothed belts and on which a solid lubrication film is formed. The body is cold-forged in a single operation by means of a punch and die. The punch has convex sections and concave sections matching the shape of the inwardly facing surface of said tooth section and also has a plurality of circumferential grooves cut into said convex sections. The die has an upper cylindrical section into which the circumferential wall section fits and a lower die section in which concave sections and convex sections are formed so that there are clearance gaps relative to said confronting convex and concave sections of the punch that are smaller than the wall thickness of the circumferential wall section. A tapered inclined face is formed between the upper cylindrical section and said lower die section so that as the circumferential section passes the tapered inclined face, it forms the circumferential wall section of the cup body into the tooth section utilizing pressure in the radial direction exerted by the inclined face.
A plurality of the reinforcing crosspieces or ribs project beyond the inner face of the tooth section in a pulley and are formed by displacing or upsetting the material of the sheet metal body into grooves provided on the punch when the circumferential wall section is forcibly pressed radially toward the center by the tapered inclined face. At the same time, while the tooth bottom section is pushed into the concave sections of the punch, the reinforcing crosspieces reduce the reduction in the material thickness because the reinforcing crosspieces or ribs obstruct the material flow axially of a pulley and thereby the reinforcing crosspieces have the effect that the tooth shape is maintained to a desired shape and the rigidity for the tension in a toothed belt is improved.
The solid lubrication film on the cup-shaped body makes material flow easily from the outwardly positioned tooth section and the inwardly positioned tooth bottom section to the intermediate tooth side section when the circumferential wall section or flange is deformed by the tapered inclined face and is pressed against the convex sections of the punch. Since the reinforcing crosspieces formed by the grooves in the punch have a height of about 0.1-0.2 mm, the tooth section can readily be disengaged from the punch after the forging operation due to the elastic deformation of the tooth section. There is, therefore, no problem in separating the work pieces from the die and punch.